This invention relates generally to tape automated bonding techniques for making electrical connections to integrated circuit devices.
Integrated circuit devices in the form of an unpackaged die or chip are typically packaged to provide electrical connections. These electrical connections allow the die or chip to be connected to other devices. While it is possible to produce very small elements in integrated circuit devices, because of the mechanical nature of the contacts, generally the contacts must be large relative to the size of the die or chip.
Therefore, it is not uncommon that the contacts on an integrated circuit package take as much surface area as the entire integrated circuit die itself. However, there is a continuing effort to reduce the size and spacing between such contacts to allow ever smaller integrated circuit packages. Such interconnection assemblies in relatively small dimensions are commonly called fine pitch assemblies.
One popular interconnection technique is a so called ball grid array (BGA). An array of solder balls may be coupled to another component by simply contacting bonding pads on one device to the solder balls on another device and heat reflowing to activate the solder. Techniques for providing very fine pitch ball grid array packages are commonly called fine pitch ball grid array (FBGA) packaging.
Thus, in one example, an integrated circuit device having a plurality of solder balls extending outwardly from the package may simply be placed on a printed circuit board. When subjected to temperature reflow, the integrated circuit device is automatically connected to the printed circuit board in a so-called surface mount technique.
Tape automated bonding or TAB provides a high speed technique for providing interconnections to chips. In effect, an interconnection layer provided in the form of a continuous adhesive tape may be secured to the die in an automated fashion. Metallic leads inside the tape layer may then be caused to contact bond pads on the die. At the same time, the other side of the tape layer may be adapted to make connections to the outside world. For example, tape layers may be coupled to solder balls. In this case, the tape may make electrical connection to bond pads on the die on one side and may couple to solder balls on the other side for connection to the outside world. A layer of traces within the tape layer may be used to connect the solder balls to the bond pads on the die.
Because of the different materials involved in making the die and the tape layers, the tape layers may have a different temperature coefficient of expansion (TCE) than the die or chip to which it is connected. This may result in failure of the interconnection and ultimately the loss of the entire packaged integrated circuit device.
Techniques are known for facilitating the relative expansion between the tape layer and the die. For example, in one known technique, the tape layer includes a cantilevered metallic beam which is deflected to make contact with bond pads on the die. Through an elaborate procedure, the beam is bent down to make contact with the die in a way which, in effect, provides a bow in the beam. This is done by pushing the beam down and axially towards its support at the same time, creating the bowed shape. In this way, the relative thermal expansion may be accounted for by bending motion within the bowed cantilevered beam.
As an analogy, the cantilevered beam is attached on two ends like a bow for a bow and arrow as indicated in FIGS. 6A and 6B. When the ends move towards each other as indicated by the arrows I and J in FIG. 6A, the beam simply deflects outwardly, as indicated by the arrow K, on the convex side of the beam to accommodate for the relative motion. Similarly, in response to a vertical displacement, indicated by the arrows L and M in FIG. 6B, the beam bows in the direction of the arrow N. Displacement of the ends of the beam, in substantially any direction, is transformed into either an increase or decrease in the bowing of the beam around the same central axis, indicated at C, of the beam.
This bowing has a beneficial effect in one sense because the stress is advantageously relieved, increasing the life of the system. However, regardless of the direction of the applied force, the response of the beam is the same. It always deflects around the same axes. This results in an increase in the amount of strain which must be withstood by the beam. In addition, the bowing tends to concentrate stresses at the points where the ends of the beam are connected to the die or the tape, thereby creating a stress riser at these locations. Unfortunately, these locations are among the most highly stressed in the entire system, increasing the possibility of failure at these locations.
Thus, there is a continuing need for better ways to relieve stress in tape bonding systems and especially for better ways to relieve stress in fine pitch assemblies.
In accordance with one aspect, a tape bonding system includes a tape and a conductive lead. The lead is situated in a surface and is secured to the tape. The lead is adapted to be bonded to a bond pad at one location and to be supported by the tape at another location. A stress relief is formed in the lead between the two locations. The stress relief is adapted to convert stress along the lead into rotation about an axis substantially transverse to the surface in which the lead is situated.
Other aspects are described in the accompanying detailed description and claims.